The present invention relates, in general, to a transconductance continuous time filter circuit and method for maintaining a wide turning range without an excessive change in bias current or device operating conditions. The invention is particularly, but not exclusively, concerned with the implementation of such a filter circuit in CMOS (Complementary Metal-Oxide Silicon) technology.
Modern mass storage data retrieval applications contain equalization circuitry to modify the recovered analog signal prior to any subsequent digital processing. Such analog equalization circuitry generally includes integrated continuous time filter circuits. Each of the integrated continuous time filter circuits generally comprises a plurality of biquadratic filters.
It is known to implement bipolar transistors in stacked active filter circuits. Where the filter must operate at a lower supply voltage, such as where a large number of stages are used in a filter such as in an integrated circuit implementation, a folded version of this architecture is preferable. In bipolar transistors, the small signal transconductance is directly proportional to the emitter current at which the device is biased. Therefore, in order to tune the filter by changing the transconductance, it is necessary to change the emitter current accordingly.
In many systems, however, it is preferable for both digital and analog parts of a system to utilize a common technology. The current dominant technology for digital circuitry is CMOS (Complementary Metal-Oxide Silicon), and hence it is desirable to provide an active filter circuit using MOS transistors in place of bipolar transistors.
However, in CMOS technology, the transconductance of a transistor operating in the saturated region is approximately proportional to the square root of the bias current. Therefore, to vary the cut-off frequency (determined by the transconductance), the bias current would need to be varied according to a square power law that would require much higher power.
It is therefore desirable to produce a continuous active filter circuit which can be implemented in CMOS technology and provides a wide turning range without requiring an excessive change in bias current or device operating conditions.
The present invention concerns a transconductance continuous time filter circuit having a wide tuning range comprising a first differential pair of transistors, at least one further pair of transistors, and at least one switch. The switch may be coupled to the at least one further pair of transistors. The switch may be turned on or off to couple or uncouple the at least one further pair of transistors from the first differential pair of transistors.